Musannaf Ibn Abi Shayba 3:4330

Abu Mu'awiyah and Hafs narrated to us, from al-A'mash, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever fears that he will not wake up at the end of the night, let him perform Witr at the beginning of the night, for the prayer at the end of the night is witnessed." Abu Mu'awiyah said: "Attended," and that is better.